Summary

The researchers developed a method using microfluidics to create bubbles coated with nanoparticles that were stable for a long period of time and had low polydispersity.

soft matter keywords: bubbles, foam, pressure, surface tension

Description

Figure 1. Depiction of how bubbles are generated.

The researchers used a microfluidic-centered approach to generating bubbles. A combination of carbon dioxide and insoluble gasses were flowed into a microfluidic channel. The production of bubbles arose from carbon dioxide's solubility in water, which followed Henry's law:

<math>[CO_2]_f = k_H P_{CO_2}</math>

Here, <math>[CO_2]_f</math> was the concentration of dissolved carbon dioxide, <math>k_H</math> was Henry's law constant and depended on the pH at which the reactions were occurring, and <math>P_{CO_2}</math> was the gas pressure. After these bubbles were created, because of the dissolution of carbon dioxide into bicarbonate <math>(HCO_3^-)</math>, the perimeter of the bubble was negatively charged. This induced the attraction and adherence of positively charged lysozyme molecules. These then attracted and were surrounded by negatively charged alginate and nanoparticles. The researchers tested several nanoparticles, including those of <math>Fe_2O_3</math>, gold, and <math>SiO_2</math>-encapsulated CdSe/ZnS.

Figure 2. Polydispersity of bubbles.

The researchers found that the polydispersity of the bubbles did not exceed 6%. They found, moreover, that they could control the size of the bubbles, ranging from 5 to 15 microns, by varying <math>Q_L</math>, the flow rate of aqueous alginate and lysozyme into the microfluidic device. Panels A - C of Figure 2 show how the bubbles shrank from their initial size but remained constant afterward; panel B is 1 hour after bubble creation, and panel C is 1000 hours later. After the drastic reduction in diameter, the bubbles remained constant in size.

The researchers then performed other characterizations of the bubbles, such as scanning transmission electron microscopy imaging and magnetic actuation of bubbles (not shown), to verify that the bubbles had been coated with nanoparticles.